The present invention relates generally to the field of semiconductor technology, and more particularly to reducing direct source to drain tunneling in field effect transistors with low effective mass channels.
Semiconductor device scaling to smaller feature sizes is facing significant challenges, such as increases in power consumption as increasing OFF-state leakage and non-scalability of the operating voltage, in pursuit of faster device performance. Traditional semiconductor design techniques, processes and materials become ineffective as physical dimensions shrink down to the nanometer regime. Increasing use of low effective mass semiconductor materials provide an increase in the maximum obtainable velocity of charge carriers such as electrons and holes. The effective mass is the mass an electron or a hole appears to have when in a solid material. The effective mass of electrons and holes in semiconducting materials, in general, is lower than the mass of a free electron. The effective mass, in some cases, may be considered a constant of a material, however, in general, the value of the effective mass depends on the application where it is used. The effective mass may be used in transport calculations, such as the transport of electrons under the influence of electric fields or carrier gradients.
Group III-V semiconductor materials provide smaller effective mass than group IV semiconductor materials and are thus, desirable for increasing performance due to a corresponding increase in electron velocity associated with lower effective mass. Group IV and group III-V refers to the location of the semiconductor element in a column of the periodic table of the elements. A group of semiconductor elements generally share similar characteristics, for example, similar physical and electrical characteristics of the outermost shell of electrons. A group III-V semiconductor is a semiconductor material that includes at least one element or semiconductor material from group III and at least one element or semiconductor material from group V of the periodic table of the elements. Group III-V semiconductor materials see increasing use in high performance semiconductor devices, particularly, in the nanometer regime.
In addition, as thinner layers are desired in the nanometer regime, an increase in electron tunneling occurs between the channel, often using III-V semiconductor materials with low effective mass, and the gate. High-k dielectric materials such as group IVb metal silicates, including hafnium and zirconium silicates and oxides, are commonly being used to reduce leakage due to electron tunneling in applications for less than fifty nanometers.
In further efforts to improve device and semiconductor material performance in the nanometer regime, bandgap engineering has been utilized. In solid-state physics, a bandgap is an energy range in a solid wherein electron states can exist. In general for semiconductor chips, bandgap refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulator materials and in semiconductor materials. The bandgap is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material and thus, is a major factor in determining the electrical conductivity of a semiconductor material. Bandgap engineering is the process of controlling or altering the bandgap of a material by controlling the composition of semiconductor alloys such as group III-V semiconductor materials. Group III-V semiconductor materials such as some alloys of GaAlAs, InGaAs, and InAlAs are often employed in bandgap engineering for advanced or nanometer regime devices. Bandgap engineering includes constructing layered semiconductor materials with alternating semiconductor compositions using techniques such as molecular beam epitaxy and metal-organic chemical vapor deposition.